1. Technical Field
The present disclosure relates generally to nanotube fabric layers and films and, more specifically, to methods for arranging nanotube elements within nanotube fabric layers and films via the application of a directional force.
2. Discussion of Related Art
Any discussion of the related art throughout this specification should in no way be considered as an admission that such art is widely known or forms part of the common general knowledge in the field.
Nanotube fabric layers and films are used in a plurality of electronic structures, and devices. For example, U.S. patent application Ser. No. 11/835,856 to Bertin et al., incorporated herein by reference in its entirety, teaches methods of using nanotube fabric layers to realize nonvolatile devices such as, but not limited to, block switches, programmable resistive elements, and programmable logic devices. U.S. Pat. No. 7,365,632 to Bertin et al., incorporated herein by reference, teaches the use of such fabric layers and films within the fabrication of thin film nanotube based resistors. U.S. patent application Ser. No. 12/066,063 to Ward et al., incorporated herein by reference in its entirety, teaches the use of such nanotube fabrics and films to form heat transfer elements within electronic devices and systems.
Through a variety of previously known techniques (described in more detail within the incorporated references) nanotube elements can be rendered conducting, non-conducting, or semi-conducting before or after the formation of a nanotube fabric layer or film, allowing such nanotube fabric layers and films to serve a plurality of functions within an electronic device or system. Further, in some cases the electrical conductivity of a nanotube fabric layer or film can be adjusted between two or more non-volatile states as taught in U.S. patent application Ser. No. 11/280,786 to Bertin et al., incorporated herein by reference in its entirety, allowing for such nanotube fabric layers and films to be used as memory or logic elements within an electronic system.
U.S. Pat. No. 7,334,395 to Ward et al., incorporated herein by reference in its entirety, teaches a plurality of methods for forming nanotube fabric layers and films on a substrate element using preformed nanotubes. The methods include, but are not limited to, spin coating (wherein a solution of nanotubes is deposited on a substrate which is then spun to evenly distribute said solution across the surface of said substrate), spray coating (wherein a plurality of nanotube are suspended within an aerosol solution which is then dispersed over a substrate), and dip coating (wherein a plurality of nanotubes are suspended in a solution and a substrate element is lowered into the solution and then removed). Further, U.S. Pat. No. 7,375,369 to Sen et al., incorporated herein by reference in its entirety, and U.S. patent application Ser. No. 11/304,315 to Ghenciu et al., incorporated herein by reference in its entirety, teach nanotube solutions well suited for forming a nanotube fabric layer over a substrate element via a spin coating process.
While there exist a number of previously known techniques for moving and orienting individual nanotube elements—atomic force microscopy probes, for example, the use of which is well known by those skilled in the art for adjusting the position of single nanotube elements in laboratory experiments and the like—there is a growing need within the current state of the art to arrange relatively large scale films and fabrics of nanotube elements for larger scale, commercial applications. For example, as the physical dimensions of nanotube fabric based electronic devices scale below twenty nanometers, there is a growing need to develop denser nanotube fabrics. That is, to form nanotube fabrics in such a way as to limit the size of—or, in some cases, substantially eliminate—gaps and voids between individual nanotube elements. In another example, within certain applications—such as, but not limited to, nanotube fabric based field effect devices, nanotube fabric based photovoltaic devices, and nanotube fabric based sensors—there is a need for nanotube fabric layers that exhibit relatively uniform physical and electrical properties. Within such applications the orientation of nanotube elements relative to each other within a film can significantly affect the overall electrical parameters of the film (such as, but not limited to, charge mobility, sheet resistance, and capacitance).
Small scale nanotube arrangement techniques (such as, but not limited to, atomic force microscopy) are typically limited to adjusting the position of a very small number of nanotubes at a time, and then typically only in the micron range. Further such laboratory based methods are not scalable or easily adapted to any large scale, commercial application. As such, such methods are not practical for the arrangement of nanotube elements in large scale films and fabrics.
A number of previously known techniques for orienting nanotube elements within a relatively large scale film involve subjecting a dispersion of nanotube elements to an electrical or mechanical field as the dispersion is deposited over a substrate layer. For example, Ma et al. (“Alignment and Dispersion of Functionalized Carbon Nanotubes in Polymer Composites Induced by an Electric Field,” Carbon 46(4):706-710 (2008)) teaches an alignment process for nanotube elements which includes applying an electrical field to a quantity of functionalized multi-walled carbon nanotubes suspended in a polymeric composite. Under the effect of the field, the functionalized nanotube will oriented themselves within the polymeric composite into a substantially uniform orientation. In another example, Merkulov et al. (“Alignment Mechanism of Carbon Nanofibers Produced by Plasma-Enhanced Chemical Vapor Deposition,” Applied Physics Letters 79:2970 (2001)) teaches a method for directing the growth of carbon nanofibers by applying an electric field during a CVD growth process. In this way, nanotube growth will tend to follow the electric field lines.
Some other previously known techniques for orienting nanotube elements within a film involve applying a mechanical force to compress vertically grown (within respect to the plane of an underlying substrate) nanotube elements into a film of substantially parallel nanotubes. For example, de Heer, et al. (Aligned Carbon Nanotube Films: Production and Optical and Electronic Properties” Science 268(5212):845-847 (1995)) teaches a method of using a Teflon or aluminum pad to compress a vertically oriented distribution of nanotube elements into a film of essentially aligned nanotube elements. Similarly, Tawfick et al. (“Flexible High-Conductivity Carbon-Nanotube Interconnects Made by Rolling and Printing” Small (Weinheiman der Bergstrasse, Germany) (2009)) teaches a method of using a roller element to pack down a distribution of vertically grown nanotube elements into a substantially aligned horizontal film.
While these related techniques do not require a mobilizing fluid vehicle (as in the methods taught by Ma and Merkulov), they do require a distribution of vertically grown nanotubes. The fabrication and use of such vertical films grown in situ can be limiting within certain applications. For example, the growth of vertical nanotube films typically requires special operation conditions (such as, but not limited to, high temperatures, certain regents, and high gas pressures), which can be undesirable or otherwise inconvenient within certain semiconductor manufacturing operations. Such conditions may be incompatible with certain substrate materials, for example. Further, the catalysts used to grow nanotubes are typically metals or metalloids, materials which can be difficult to remove within high purity applications. Further, in situ growth of films limit the ability to form blends of nanotube formulations—for example, combinations of semiconducting and metallic nanotubes, single walled and multi walled nanotubes, or nanotubes mixed with other materials like buckyballs, silica, or other material particles. Further still, the roughness of vertically grown films is dictated by the density and uniformity of the vertical tubes as grown without additional liquid processing to enhance tube association. Such limitations within the growth of vertical nanotube films reduce their effectiveness and limit their applicability in large scale, commercial applications.
While these and other similar previously known methods provide some means of aligning or otherwise orienting nanotube elements, they are limited in that they require either wet suspensions of nanotube elements or nanotube elements grown in vertical orientations. Within many applications, these limitations will substantially limit the effectiveness of these techniques in commercial applications. Further, these previously known techniques will tend to limit the orientation of the aligned nanotube elements along a single direction. As such, there is a need for an efficient and relatively uncomplicated method of arranging nanotube elements within a dry nanotube fabric (for example, a nanotube fabric formed by spin coating a nanotube application solution over a substrate). Further, there is a need for a method of arranging nanotube elements within a nanotube fabric according to a preselected orientation (which may include nanotube arrangement along multiple directions).